President Trump said on Monday that he would oppose any effort to reduce the amount of pretax income that American workers can save in 401(k) retirement accounts, effectively killing an idea that Republicans were mulling as a way to help pay for a $1.5 trillion tax cut.

The directive, issued via Twitter, underscored a growing fear among Republicans and business lobbyists that Mr. Trump’s bully-pulpit whims could undermine the party’s best chance to pass the most sweeping rewrite of the tax code in decades.

.. Mr. Trump hosted House Republicans in the Rose Garden to celebrate passage of a bill to repeal the Affordable Care Act, only to call the same bill “mean” later.

.. Mr. Trump “can shift on a dime, and he has many unformed policy positions,” said Representative Charlie Dent, Republican of Pennsylvania. “We have to worry about him shifting positions.”

.. Republicans are also discussing whether to raise the top income tax rate for top earners and jettisoning a proposal to eliminate the estate tax.

.. Reducing 401(k) contribution limits would force retirement savers to pay more in taxes today, as they sock away money, but less in the future, when they begin withdrawing retirement funds.

.. “Trump engages in fits and starts and then undermines his side’s negotiating positions half the time,”

.. “He doesn’t care about the details of tax reform, or Obamacare repeal and replace, and thus inserting himself into the negotiations has been largely counterproductive to date.”

.. What privately alarms even supporters of Mr. Trump’s on Capitol Hill is the possibility that he cannot stomach unpopular issues. Such supporters often point to President Ronald Reagan’s championing of the 1986 tax reform bill, which eliminated many business tax preferences, as a contrast to Mr. Trump’s recent actions.

That same dynamic could also be a benefit for industry groups, which may be able to rout a proposal by portraying a change as going against public sentiment.

.. Many observers in Washington say Mr. Trump has frequently promised more than he can deliver, again pointing to the health care debate, where he campaigned on a promise to provide better, lower-cost coverage to more Americans.

Others point to Mr. Trump’s promises on the budget, where he vowed to

reduce the federal deficit and

pay down the national debt, while

both cutting taxes and

leaving two major safety-net spending programs, Medicare and Social Security, untouched.

.. “the president has never met a hard choice he was willing to make.”